Nonvolatile semiconductor memory device

ABSTRACT

When selectively erasing one sub-block, a control circuit applies, in a first sub-block, a first voltage to bit lines and a source line, and applies a second voltage smaller than the first voltage to the word lines. Then, the control circuit applies a third voltage lower than the first voltage by a certain value to a drain-side select gate line and a source-side select gate line, thereby performing the erase operation in the first sub-block. The control circuit applies, in a second sub-block existing in an identical memory block to the selected sub-block, a fourth voltage substantially identical to the first voltage to the drain side select gate line and the source side select gate line, thereby not performing the erase operation in the second sub-block.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of priorityunder 35 U.S.C. § 120 from U.S. application Ser. No. 16/122,945 filedSep. 6, 2018, which is a continuation of U.S. application Ser. No.15/915,701 filed Mar. 8, 2018 (now U.S. Pat. No. 10,090,054 issued Oct.2, 2018), which is a continuation of U.S. application Ser. No.15/810,489 filed Nov. 13, 2017 (now U.S. Pat. No. 9,953,713 issued Apr.24, 2018), which is a continuation of U.S. application Ser. No.15/648,530 filed Jul. 13, 2017 (now U.S. Pat. No. 9,852,797 issued Dec.26, 2017), which is a continuation of U.S. application Ser. No.15/403,339 filed Jan. 11, 2017 (now U.S. Pat. No. 9,747,988 issued Aug.29, 2017), which is a continuation of U.S. application Ser. No.15/275,614 filed Sep. 26, 2016 (now U.S. Pat. No. 9,595,337 issued Mar.14, 2017), which is a continuation of U.S. application Ser. No.14/842,382 filed Sep. 1, 2015 (now U.S. Pat. No. 9,484,105 issued Nov.1, 2016), which is a continuation of U.S. application Ser. No.14/493,413 filed Sep. 23, 2014 (now U.S. Pat. No. 9,159,431 issued Oct.13, 2015), which is a continuation of U.S. application Ser. No.14/098,237 filed Dec. 5, 2013 (now U.S. Pat. No. 8,861,274 issued Oct.14, 2014), which is a continuation of U.S. application Ser. No.13/970,689 filed Aug. 20, 2013 (now U.S. Pat. No. 8,649,227 issued Feb.11, 2014), which is a continuation of U.S. application Ser. No.13/149,139 filed May 31, 2011 (now U.S. Pat. No. 8,537,615 issued Sep.17, 2013), and claims the benefit of priority under 35 U.S.C. § 119 fromJapanese Patent Application No. 2010-264872 filed Nov. 29, 2010, theentire contents of each of which are incorporated herein by reference.

FIELD

Embodiments described herein relate to a nonvolatile semiconductormemory device.

BACKGROUND

Conventionally, an LSI is formed by integrating elements in atwo-dimensional plane on a silicon substrate. Generally, a storagecapacity of memory is increased by reducing dimensions of(miniaturizing) an element. However, in recent years, even thisminiaturization is becoming difficult in terms of cost and technology.Improvements in photolithographic technology are necessary forminiaturization, but costs required in lithographic processes arerapidly increasing. In addition, even if miniaturization is achieved, itis expected that physical limitations such as those of withstand voltagebetween elements are encountered, unless the drive voltage and so on arescaled. Moreover, the reduction in distance between memory elements thataccompanies miniaturization causes an increase in adverse effects due tocapacitive coupling between each of the memory elements duringoperations. In other words, there is a high possibility that operationas a device becomes difficult. Accordingly, in recent years, there areproposed many nonvolatile semiconductor memory devices (stacking-typenonvolatile semiconductor memory devices) in which memory cells aredisposed three-dimensionally in order to increase a degree ofintegration of memory.

One conventional semiconductor memory device in which memory cells aredisposed three-dimensionally uses a transistor with a cylindrical columntype structure. The semiconductor memory device using the transistorwith a cylindrical column type structure is provided with multiplelayers of polysilicon configuring a gate electrode, and a pillar-shapedcolumnar semiconductor. The columnar semiconductor is disposed topenetrate the polysilicon layers and has a memory cell formed atportions of intersection with those polysilicon layers. In this memorycell, the columnar semiconductor functions as a channel (body) portionof a transistor. A vicinity of the columnar semiconductor is providedwith a charge storage layer, each sandwiching a tunnel insulating layerand configured to store a charge. Furthermore, a block insulating layeris formed in a vicinity of the charge storage layer. The polysilicon,columnar semiconductor, tunnel insulating layer, charge storage layerand block insulating layer configured in this manner form a memorystring of series-connected memory cells.

An erase operation in this kind of conventional semiconductor memorydevice in which memory cells are disposed three-dimensionally isperformed in units of a memory block, the memory block being an assemblyof memory strings to which word lines are commonly connected. In aconventional stacking-type semiconductor memory device, there is aproblem that, along with an increase in the number of layers, there isan increase in the number of word lines commonly connected to aplurality of memory strings in one memory block, this leading to anincrease in the number of memory cells included in one memory block.Consequently, there is desired a stacking-type semiconductor memorydevice which, in addition to being capable of the erase operation on amemory block basis, is also capable of an erase operation to selectivelyerase only a part of the memory cells in a memory block.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing an overall configuration of anonvolatile semiconductor memory device in accordance with a firstembodiment.

FIG. 2 is a schematic perspective view of a memory cell array AR1 inFIG. 1.

FIG. 3A is an equivalent circuit diagram showing a circuit configurationof the memory cell array AR1 in FIG. 1.

FIG. 3B is a schematic cross-sectional view of a memory block MB in thememory cell array AR1 in FIG. 1.

FIG. 3C is a schematic cross-sectional view of inside another memorycell array.

FIG. 4 is a schematic cross-sectional view of a memory unit MU in onememory block MB.

FIG. 5 is a plan view of one memory block MB.

FIG. 6 shows an erase operation in the first embodiment.

FIG. 7 shows the erase operation in the first embodiment.

FIG. 8 shows the erase operation in the first embodiment.

FIG. 9A is one example of a charge pump circuit and a voltage valueadjusting circuit optimal for generating various voltages in the firstembodiment.

FIG. 9B is one example of a row decoder 2A employed in the firstembodiment.

FIG. 10 is a circuit diagram of an overall configuration of anonvolatile semiconductor memory device in accordance with a secondembodiment.

FIG. 11 is a schematic perspective view of a memory cell array AR1 inFIG. 10.

FIG. 12 is a schematic cross-sectional view of one memory block MB inthe memory cell array AR1 in FIG. 10.

FIG. 13 is a schematic cross-sectional view of one memory unit MU in onememory block MB.

FIG. 14 shows an erase operation in the second embodiment.

FIG. 15A shows the erase operation in the second embodiment.

FIG. 15B shows the erase operation in the second embodiment.

FIG. 16A shows an erase operation in a modified example in the secondembodiment.

FIG. 16B shows the erase operation in the modified example in the secondembodiment.

FIG. 17A is one example of a charge pump circuit and a voltage valueadjusting circuit optimal for generating various voltages in the secondembodiment.

FIG. 17B is one example of a row decoder 2A employed in the secondembodiment.

FIG. 18A is a circuit diagram of an overall configuration of anonvolatile semiconductor memory device in accordance with a thirdembodiment.

FIG. 18B is one example of a row decoder 2A employed in the thirdembodiment.

FIG. 19A is a circuit diagram of an overall configuration of anonvolatile semiconductor memory device in accordance with a fourthembodiment.

FIG. 19B is a circuit diagram of an overall configuration of a modifiedexample of the nonvolatile semiconductor memory device in accordancewith the fourth embodiment.

FIG. 20 is a schematic perspective view of a memory cell array AR1 inFIG. 19.

FIG. 21 is a schematic cross-sectional view of one memory block MB inthe memory cell array AR1 in FIG. 19.

FIG. 22A shows an operation in a nonvolatile semiconductor memory devicein accordance with a fifth embodiment.

FIG. 22B shows an operation in a modified example of the nonvolatilesemiconductor memory device in accordance with the fifth embodiment.

FIG. 23 is one example of a charge pump circuit optimal for generatingvarious voltages in the fifth embodiment.

FIG. 24 is a circuit diagram of an overall configuration of anonvolatile semiconductor memory device in accordance with a sixthembodiment.

FIG. 25 is a schematic perspective view of a memory cell array AR1 inFIG. 24.

FIG. 26 is a schematic cross-sectional view of a memory block MB in thememory cell array AR1 in FIG. 24.

FIG. 27 shows an erase operation in the sixth embodiment.

FIG. 28 shows the erase operation in the sixth embodiment.

FIG. 29 is one example of a charge pump circuit and a voltage valueadjusting circuit optimal for generating various voltages in the sixthembodiment.

FIG. 30 is a circuit diagram of an overall configuration of anonvolatile semiconductor memory device in accordance with a seventhembodiment.

FIG. 31 is a schematic perspective view of a memory cell array AR1 inFIG. 30.

FIG. 32 shows an erase operation in the seventh embodiment.

FIG. 33 shows the erase operation in the seventh embodiment.

DETAILED DESCRIPTION

A nonvolatile semiconductor memory device in an embodiment describedhereinafter comprises a memory cell array including a plurality ofmemory blocks. Arranged in each of the plurality of memory blocks are aplurality of memory strings disposed in a matrix and each configuredfrom a plurality of electrically rewritable memory transistors connectedin series. One end of a drain side select transistor is connected to afirst end of the memory string, and one end of a source side selecttransistor is connected to a second end of the memory string. Aplurality of word lines are disposed so as to be commonly connected tothe plurality of memory strings disposed in one of the plurality ofmemory blocks. In addition, a plurality of bit lines each extends in afirst direction and is commonly connected to the other end of the drainside select transistor in the plurality of memory blocks. A source lineis connected to the other end of the source side select transistor. Adrain side select gate line is disposed along a second direction as alonger direction thereof and so as to commonly connect a gate of thedrain side select transistor aligned in the second direction, the seconddirection being orthogonal to the first direction. A source side selectgate line is disposed along the second direction as a longer directionthereof and so as to commonly connect a gate of the source side selecttransistor aligned in the second direction. A control circuit controls avoltage applied to the plurality of memory blocks.

Each of the plurality of memory strings comprises: a columnarsemiconductor layer including a columnar portion extending in aperpendicular direction with respect to a substrate, the columnarsemiconductor layer being configured to function as a body of the memorytransistors; a charge storage layer formed so as to surround a sidesurface of the columnar portion and configured to allow storage of acharge; and a word line conductive layer formed so as to surround theside surface of the columnar portion with the charge storage layerinterposed therebetween, the word line conductive layer being configuredto function as a gate of the memory transistors and as the word lines. Aplurality of the memory strings that are connected to a plurality of thedrain side select transistors and a plurality of the source side selecttransistors which are commonly connected to one of the drain side selectgate lines and one of the source side select gate lines configure onesub-block. For execution of an erase operation of selectively erasing atleast one of the sub-blocks in the memory blocks, the control circuit isconfigured to apply, in a first sub-block as a selected sub-block, afirst voltage to the bit lines and the source line, and a second voltagesmaller than the first voltage to the word lines. Then, the controlcircuit applies a third voltage lower than the first voltage by acertain value to the drain side select gate line and the source sideselect gate line, thereby performing the erase operation in the firstsub-block. On the other hand, the control circuit applies, in a secondsub-block as an unselected sub-block existing in an identical memoryblock to the selected sub-block, a fourth voltage substantiallyidentical to the first voltage to the drain side select gate line andthe source side select gate line, thereby not performing the eraseoperation in the second sub-block.

Embodiments of a nonvolatile semiconductor memory device in accordancewith the present invention are described below with reference to thedrawings.

First Embodiment

First, an overall configuration of a nonvolatile semiconductor memorydevice in accordance with a first embodiment is described with referenceto FIG. 1. FIG. 1 is a circuit diagram of the nonvolatile semiconductormemory device in accordance with the first embodiment.

As shown in FIG. 1, the nonvolatile semiconductor memory device inaccordance with the first embodiment includes a memory cell array AR1and, disposed in a periphery of that memory cell array AR1, row decoders2A and 2B, a sense amplifier circuit 3, a column decoder 4, and acontrol circuit AR2.

As shown in FIG. 1, the memory cell array AR1 is configured having aplurality of memory strings MS arranged in a matrix, each of the memorystrings MS having electrically rewritable memory transistors MTr1-8(memory cells) connected in series. The control circuit AR2 isconfigured by various kinds of control circuits configured to control avoltage applied to gates of the memory transistors MTr (MTr1-8), and soon.

The row decoders 2A and 2B are disposed on the left side and right side,respectively, of the memory cell array AR1, and, in accordance with anaddress signal from the control circuit AR2, drive word lines WL, selectgate lines SGD and SGS, and a back gate line BG. The column decoder 4selects an address where read and write are to be performed, inaccordance with an address signal supplied from the control circuit AR2.The sense amplifier circuit 3 determines data stored in memory cellsduring a read operation. In addition, the sense amplifier 3 drives bitlines BL and a source line SL in accordance with an address signalsupplied from the control circuit AR2 via the column decoder 4.

The control circuit AR2 comprises: a driver 201 configured to drive theword lines WL, the select gate lines SGD and SGS, and the back gate lineBG; a driver 202 configured to drive the source line SL; a charge pumpcircuit 203 configured to boost a power supply voltage to a certainboost voltage; and an address decoder 204.

The control circuit AR2 executes a write operation of data to the memorytransistors MTr, an erase operation of data in the memory transistorsMTr, and the read operation of data from the memory transistors MTr.Voltages applied to the memory transistors MTr during the writeoperation and the read operation are substantially similar to those in aconventional stacking-type flash memory.

As shown in FIG. 1, the memory cell array AR1 includes memory blocks MB.Each of the memory blocks MB includes memory units MU arranged in an ncolumn by two row matrix, for example. Each of the memory units MUcomprises: the memory string MS; a source side select transistor SSTr2connected to a source side of the memory string MS; a drain side selecttransistor SDTr2 connected to a drain side of the memory string MS; anda back gate transistor BTr. Note that, in the example shown in FIG. 1, afirst row of memory units MU is referred to as a sub-block SB1, and asecond row of memory units is referred to as a sub-block SB2. In FIG. 1,the case is described where there are two sub-blocks SB1 and SB2 in onememory block MB. However, naturally, the present embodiment is notlimited to this case, and there may be three or more sub-blocks providedto one memory block MB.

The m memory blocks MB share identical bit lines BL. That is, each ofthe bit lines BL extends in a column direction shown in FIG. 1 and isconnected to the plurality of memory units MU (drain side selecttransistors SDTr2) arranged in a line in the column direction in each ofthe m memory blocks MB. Two memory units MU aligned in the columndirection in each of the memory blocks MB are commonly connected to anidentical bit line BL.

In addition, 2×n memory units MU in each of the memory blocks MB sharethe word lines WL and the back gate line BG. Moreover, the n memoryunits MU aligned in a row direction (that is, the memory units MU in onesub-block) share a select gate line SGD and a select gate line SGS. Thatis, a plurality of the memory strings MS connected to a plurality of thedrain side select transistors SDTr and a plurality of the source sideselect transistors SSTr commonly connected to one of the drain sideselect gate lines SGD and one of the source side select gate lines SGSconfigure one sub-block.

As shown in a schematic perspective view of FIG. 2, the memory cellarray AR1 is configured having the electrically data-storing memorytransistors MTr arranged in a three-dimensional matrix. That is, thememory transistors MTr, in addition to being arranged in a matrix in ahorizontal direction, are also arranged in a stacking direction(vertical direction with respect to a substrate). The plurality ofmemory transistors MTr1-8 aligned in the stacking direction areconnected in series to configure the aforementioned memory string MS. Todetermine select/unselect of the memory string MS, the drain side selecttransistor SDTr2 is connected to one end of the memory string MS and thesource side select transistor SSTr2 is connected to the other end of thememory string MS. This memory string MS is arranged having the stackingdirection as a longer direction thereof. Note that details of a stackingstructure are described later.

Next, a circuit configuration of the memory cell array AR1 is describedspecifically with reference to FIG. 3A. FIG. 3A is an equivalent circuitdiagram of the memory cell array AR1.

As shown in FIG. 3A, the memory cell array AR1 includes a plurality ofthe memory units MU arranged in a matrix in the row direction and thecolumn direction. In the memory block MB, a plurality of commonlyconnected memory units MU are provided to one bit line BL. Each of thememory units MU includes the memory string MS, the source side selecttransistor SSTr2, and the drain side select transistor SDTr2. The memoryunits MU are arranged in a matrix in the row direction and the columndirection.

The memory string MS is configured by the memory transistors MTr1-8 andthe back gate transistor BTr connected in series. The memory transistorsMTr1-4 are connected in series in the stacking direction. Similarly, thememory transistors MTr5-8 are also connected in series in the stackingdirection. The memory transistors MTr1-8 are configured to have athreshold voltage changed with an amount of charge stored in a chargestorage layer. Changing the threshold voltage allows data retained inthe memory transistors MTr1-8 to be rewritten. The back gate transistorBTr is connected between the lowermost layer memory transistors MTr4 andMTr5. The memory transistors MTr1-MTr8 and the back gate transistor BTrare thus connected in a U shape in a cross-section in the columndirection. A drain of the source side select transistor SSTr2 isconnected to one end of the memory string MS (a source of the memorytransistor MTr8). A source of the drain side select transistor SDTr2 isconnected to the other end of the memory string MS (a drain of thememory transistor MTr1).

Gates of the 2×n memory transistors MTr1 in one memory block MB arecommonly connected to a single word line WL1 extending in the rowdirection. Similarly, gates of the 2×n memory transistors MTr2-MTr8 arecommonly connected to respective single word lines WL2-WL8 extending inthe row direction, respectively. Moreover, gates of the 2×n back gatetransistors BTr arranged in a matrix in the row direction and the columndirection are commonly connected to a back gate line BG.

Gates of the n source side select transistors SSTr2 arranged in a linein the row direction are commonly connected to a single source sideselect gate line SGS2 extending in the row direction. Moreover, sourcesof the source side select transistors SSTr2 are connected to the sourceline SL extending in the row direction.

Gates of the n drain side select transistors SDTr2 arranged in a line inthe row direction are commonly connected to a single drain side selectgate line SGD2 extending in the row direction. Drains of the drain sideselect transistors SDTr2 are connected to the bit line BL extending inthe column direction.

Next, the stacking structure of the nonvolatile semiconductor memorydevice in accordance with the first embodiment is described withreference to FIGS. 3B, 4, and 5. FIG. 3B is a schematic column-directioncross-sectional view of the memory block MB. In addition, FIG. 4 is aschematic cross-sectional view of one memory unit MU, and FIG. 5 is aplan view of the memory block MB.

As shown in FIG. 3B, the memory cell array AR1 includes, on a substrate10, a back gate transistor layer 20, a memory transistor layer 30, aselect transistor layer 40, and a wiring layer 50. The back gatetransistor layer 20 functions as the back gate transistor BTr. Thememory transistor layer 30 functions as the memory transistors MTr1-MTr8(memory string MS). The select transistor layer 40 functions as thesource side select transistor SSTr2 and the drain side select transistorSDTr2. The wiring layer 50 functions as the source line SL and the bitline BL.

As shown in FIG. 4, the back gate transistor layer 20 includes a backgate conductive layer 21. The back gate conductive layer 21 functions asthe back gate line BG and also functions as a gate of the back gatetransistor BTr.

The back gate conductive layer 21 is formed so as to extendtwo-dimensionally in the row direction and the column direction parallelto the substrate 10. The back gate conductive layer 21 is divided on amemory block MB basis. The back gate conductive layer 21 is configuredby polysilicon (poly-Si).

As shown in FIG. 4, the back gate transistor layer 20 includes a backgate hole 22. The back gate hole 22 is formed so as to dig out the backgate conductive layer 21. The back gate hole 22 is formed in asubstantially rectangular shape long in the column direction as viewedfrom an upper surface. The back gate hole 22 is formed in a matrix inthe row direction and the column direction.

As shown in FIG. 4, the memory transistor layer 30 is formed in a layerabove the back gate transistor layer 20. The memory transistor layer 30includes word line conductive layers 31 a-31 d. The word line conductivelayers 31 a-31 d respectively function as the word lines WL1-WL8 andalso as gates of the memory transistors MTr1-MTr8.

The word line conductive layers 31 a-31 d are stacked sandwichinginterlayer insulating layers (not shown) between them. The word lineconductive layers 31 a-31 d are formed so as to extend with the rowdirection as a longer direction thereof and having a certain pitch inthe column direction. The word line conductive layers 31 a-31 d areconfigured by polysilicon (poly-Si).

As shown in FIG. 3B, the memory transistor layer includes a memory hole32. The memory hole 32 is formed so as to penetrate the word lineconductive layers 31 a-31 d and the interlayer insulating layers notshown. The memory hole 32 is formed so as to align with an end vicinityin the column direction of the back gate hole 22.

Note that FIG. 3B shows an example where two memory strings MS alignedin a bit line BL direction are commonly connected to the same word lineconductive layers 31 a-31 d. However, as shown in FIG. 3C, aconfiguration may also be adopted in which memory strings MS aligned inthe bit line BL direction are connected to word line conductive layers31 a-31 d divided from each other on a memory string MS basis.

In addition, as shown in FIG. 4, the back gate transistor layer 20 andthe memory transistor layer 30 include a memory gate insulating layer 33and a memory semiconductor layer 34. The memory semiconductor layer 34functions as a body of the memory transistors MTr1-MTr8 (memory stringMS).

As shown in FIG. 4, the memory gate insulating layer 33 is formed with acertain thickness on a side surface of the back gate hole 22 and thememory hole 32. The memory gate insulating layer 33 includes a blockinsulating layer 33 a, a charge storage layer 33 b, and a tunnelinsulating layer 33 c. Storing of a charge by the charge storage layer33 b causes the threshold voltage of the memory transistors MTr1-8 tochange, thus allowing data retained by the memory transistors MTr to berewritten.

As shown in FIG. 4, the block insulating layer 33 a is formed with acertain thickness on the side surface of the back gate hole 22 and thememory hole 32. The charge storage layer 33 b is formed with a certainthickness on a side surface of the block insulating layer 33 a. Thetunnel insulating layer 33 c is formed with a certain thickness on aside surface of the charge storage layer 33 b. The block insulatinglayer 33 a and the tunnel insulating layer 33 c are configured bysilicon oxide (SiO₂). The charge storage layer 33 b is configured bysilicon nitride (SiN).

The memory semiconductor layer 34 is formed so as to be in contact witha side surface of the tunnel insulating layer 33 c. The memorysemiconductor layer is formed so as to fill the back gate hole 22 andthe memory hole 32. The memory semiconductor layer 34 is formed in a Ushape as viewed from the row direction. The memory semiconductor layer34 includes a pair of columnar portions 34 a extending in the verticaldirection with respect to the substrate 10 and a joining portion 34 bconfigured to join lower ends of the pair of columnar portions 34 a. Thememory semiconductor layer 34 is configured by polysilicon (poly-Si).

Expressing the above-described configuration of the back gate transistorlayer 20 in other words, the memory gate insulating layer 33 is formedso as to surround the joining portion 34 b. The back gate conductivelayer 21 is formed so as to surround the joining portion 34 b with thememory gate insulating layer 33 interposed therebetween. In addition,expressing the above-described configuration of the memory transistorlayer 30 in other words, the memory gate insulating layer 33 is formedso as to surround the columnar portion 34 a. The word line conductivelayers 31 a-31 d are formed so as to surround the columnar portion 34 awith the memory gate insulating layer 33 interposed therebetween.

As shown in FIG. 3B, the select transistor layer 40 includes a sourceside conductive layer 45 a and a drain side conductive layer 45 b. Thesource side conductive layer 45 a functions as the source side selectgate line SGS2 and also functions as a gate of the source side selecttransistor SSTr2. The drain side conductive layer 45 b functions as thedrain side select gate line SGD2 and also functions as a gate of thedrain side select transistor SDTr2.

The source side conductive layer 45 a is formed so as to surround asemiconductor layer 48 a, and the drain side conductive layer 45 b,which is in the same layer as the source side conductive layer 45 a, isformed similarly so as to surround a semiconductor layer 48 b. Thesource side conductive layer 45 a and the drain side conductive layer 45b are configured by polysilicon (poly-Si).

As shown in FIG. 4, the select transistor layer 40 includes a sourceside hole 46 a and a drain side hole 46 b. The source side hole 46 a isformed so as to penetrate the source side conductive layer 45 a. Thedrain side hole 46 b is formed so as to penetrate the drain sideconductive layer 45 b. The source side hole 46 a and the drain side hole46 b are each formed at a position aligning with the memory hole 32.

As shown in FIG. 4, the select transistor layer 40 includes a sourceside gate insulating layer 47 a, a source side columnar semiconductorlayer 48 a, a drain side gate insulating layer 47 b, and a drain sidecolumnar semiconductor layer 48 b. The source side columnarsemiconductor layer 48 a functions as a body of the source side selecttransistor SSTr2. The drain side columnar semiconductor layer 48 bfunctions as a body of the drain side select transistor SDTr2.

Note that a distance Dsm between the source side conductive layer 45 aor drain side conductive layer 45 b and the word line conductive layer31 d is, for example, about two to three times a distance Dmm betweenadjacent word line conductive layers 31 a-31 d. This is to prevent afalse erase operation. That is, during the erase operation, as describedlater, a high voltage is applied to the source side conductive layer 45a or drain side conductive layer 45 b, while a ground voltage Vss isapplied to the word line conductive layers 31 a-31 d. In this case, thecolumnar semiconductor layer 48 a or 48 b directly below the source sideconductive layer 45 a or drain side conductive layer 45 b rises close toan erase voltage Vera due to capacitive coupling, while a potential ofthe columnar portion 34 a directly below the word line conductive layer31 d remains at substantially 0 V. Therefore, if the distance betweenthe source side conductive layer 45 a or drain side conductive layer 45b and the word line conductive layer 31 d is short, a strong electricfield is generated between the columnar semiconductor layer 48 a or 48 bdirectly below the source side conductive layer 45 a or drain sideconductive layer 45 b and the columnar portion 34 a directly below theword line conductive layer 31 d. This causes a GIDL current to begenerated, whereby a false erase operation of data sometimes occurs inan unselected memory block. Consequently, the distance Dsm between thesource side conductive layer 45 a or drain side conductive layer 45 band the word line conductive layer 31 d must be set larger than thedistance Dmm between adjacent word line conductive layers 31 a-31 d.

As shown in FIG. 3B, the wiring layer 50 is formed in a layer above theselect transistor layer 40. The wiring layer 50 includes a source linelayer 51 and a bit line layer 52. The source line layer 51 functions asthe source line SL. The bit line layer 52 functions as the bit line BL.

The source line layer 51 is formed in a plate-like shape extending inthe row direction. The source line layer 51 is formed so as to be incontact with upper surfaces of pairs of the source side columnarsemiconductor layers 48 a adjacent in the column direction. The bit linelayer 53, which is in contact with an upper surface of the drain sidecolumnar semiconductor layer 48 b, is formed in stripes extending in thecolumn direction and having a certain pitch in the row direction. Thesource line layer 51 and the bit line layer 52 are configured by a metalsuch as tungsten (W), copper (Cu), or aluminum (Al).

Next, shapes of the word line conductive layers 31 a-31 d are describedin detail with reference to FIG. 5. FIG. 5 is a top view showing theword line conductive layer 31 a. FIG. 5 illustrates the shape of theword line conductive layer 31 a only as an example, since the word lineconductive layers 31 b-31 d have substantially the same shapes as theword line conductive layer 31 a.

As shown in FIG. 5, the word line conductive layer 31 a is formed in acomb tooth shape as viewed from a vertical direction. The word lineconductive layer 31 a have a plurality of straight portions 351 a and352 a configured to surround a plurality of the columnar semiconductorlayers 34 a aligned in the row direction; and a straight portion 351 band 352 b configured to join ends of the plurality of straight portions351 a and 352 a. In this way, the word lines connected to memory stringsMS aligned in the bit line direction are commonly connected to eachother on a memory block basis. This is because there is a need to reducethe number of metal wiring lines for connecting signals of the wordlines WL, select gate lines SGD and SGS, and back gate line BG to thelikes of the row decoder in the peripheral circuit section.

In FIG. 5, a reference numeral 34 a′ denotes a dummy columnarsemiconductor layer that is not used as a memory string MS. Note that,in the case of a configuration as in FIG. 3C, the dummy columnarsemiconductor layer 34 a′ is not necessary.

Next, the erase operation in the nonvolatile semiconductor memory devicein accordance with the present embodiment is described with reference toFIGS. 6-8. FIGS. 6 and 7 show an equivalent circuit diagram of thememory cell array AR1 and voltages applied to each part. FIG. 8 is atiming chart showing timing of application of voltages. Now, it isassumed that, of the two sub-blocks in one memory block MB, thesub-block SB1 is selectively set as erase target, and the eraseoperation executed in a sub-block unit on this sub-block SB1. At thistime, the sub-block SB2 is not subject to erase, and erase of data inthe memory cells in this sub-block SB2 is prohibited. The two sub-blocksSB1 and SB2 are both connected to identical bit lines BL, source lineSL, and word lines WL, but each has a separate drain side select gateline SGD2 and source side select gate line SGS2. Note that in thedescription below, the select gate lines SGD2 and SGS2 in the sub-blockSB1 are referred to as SGD21 and SGS21; similarly, the select gate linesSGD2 and SGS2 in the sub-block SB2 are referred to as SGD22 and SGS22.

As shown in FIG. 8, in the sub-block SB1 selected as erase target, attime t1, the bit lines BL and source line SL are each set to the erasevoltage Vera (about 20 V). Meanwhile, the word lines WL are applied withthe ground voltage Vss (0 V). Then, at time t3, the drain side selectgate line SGD21 and the source side select gate line SGS21 are eachapplied with a voltage Vera-ΔV which is lower than the voltage Vera byabout a voltage ΔV (for example, 5-8 V). This causes a GIDL (GateInduced Drain Leakage) current to be generated at an end of the drainside select transistor SDTr2 on the bit line BL side and an end of thesource side select transistor SSTr2 on the source line SL side in thesub-block SB1 (refer to FIG. 7), whereby the voltage Vera applied to thebit lines BL and source line SL is transferred to the body of the memoryunits MU in the sub-block SB1. This causes the erase operation in thesub-block SB1 to be executed as a result of a potential differencebetween the voltage Vera of the body and the voltage Vss of the wordlines WL.

On the other hand, in the unselected erase-prohibited sub-block SB2, attime t1, the bit lines BL and source line SL, since they are shared withthe sub-block SB1, are set to the erase voltage Vera (about 20 V).However, at time t2, the drain side select gate line SGD22 and thesource side select gate line SGS22 are applied with a voltage Vera′substantially identical to the erase voltage Vera. As a result, a highvoltage is not applied between the source line SL and source side selectgate line SGS and between the bit lines BL and drain side select gateline SGD, whereby generation of a GIDL current is prevented.

FIG. 9A is one example of a charge pump circuit and a voltage valueadjusting circuit optimal for generating various voltages in the presentembodiment. An oscillator 101 generates a clock signal, and a chargepump circuit 102 is inputted with this clock signal and boosts a powersupply voltage Vdd to the erase voltage Vera. Voltage values of thevoltage Vera′ and Vera-ΔV are adjusted by a voltage value adjustingcircuit 103 configured having diode-connected transistors connected inseries. In addition, a voltage determining circuit configured by adifferential amplifier 106 and splitting resistances 107 and 108 judgeswhether the voltage Vera has risen to a certain value or not, and stopsoperation of the oscillator 101 based on an output signal of thedifferential amplifier 106.

Note that in the selected memory block, the above-mentioned voltage issupplied to the select gate lines SGD2 and SGS2; however, in theunselected memory block, it is preferable for the select gate lines SGD2and SGS2 to be maintained in the floating state. One example of the rowdecoder 2A for performing such voltage control is shown in FIG. 9B (therow decoder 2B has a substantially similar configuration, hence only therow decoder 2A is described). This row decoder 2A includes an addressdetermining circuit 111 and a transfer transistor group 112. The addressdetermining circuit 111 turns on a transfer transistor 112 a configuredto switch supply of the voltage Vera′ or Vera-ΔV in the selected block,based on a block address signal Block Adrs. On the other hand, in theunselected block, a gate of a transfer transistor 112 b configured tosupply the power supply voltage Vdd is supplied with the voltage Vdd,whereby the select gate lines SGD2 and SGS2 are charged to a powersupply voltage Vdd-Vth. Subsequently, when the bit lines BL and sourceline SL rise to the voltage Vera, the voltage of the select gate linesSGD2 and SGS2 rise due to capacitive coupling, thereby causing thetransfer transistor 112 b to be turned off. As a result, the select gatelines SGD2 and SGS2 attain the floating state.

Second Embodiment

Next, a nonvolatile semiconductor memory device in accordance with asecond embodiment is described.

FIG. 10 is a circuit diagram of an overall configuration of thenonvolatile semiconductor memory device in accordance with the secondembodiment. FIG. 11 is a schematic perspective view of a memory cellarray AR1 in the nonvolatile semiconductor memory device in accordancewith the second embodiment. Note that configurations similar to those inthe first embodiment are assigned with identical symbols to the firstembodiment and detailed descriptions thereof are omitted below.

In this embodiment, the memory unit MU, as well as comprising the selecttransistors SDTr2 and SSTr2 connected to the bit lines BL and sourceline SL, also comprises separate select transistors SDTr1 and SSTr1connected in series to the select transistors SDTr2 and SSTr2. Theselect transistors SDTr1 and SSTr1 are connected between the selecttransistors SDTr2 and SSTr2 and the memory string MS. The reason forproviding these two series-connected select transistors in this way isto prevent a GIDL current from being generated in the unselected blockdue to the potential difference between the select gate lines SGD2 orSGS2 and the word lines WL as previously mentioned. Hereinafter, theselect transistors SDTr2 and SSTr2 are referred to as ‘second drain sideselect transistor SDTr2’ and ‘second source side select transistorSSTr2’; and the select transistors SDTr1 and SSTr1 are referred to as‘first drain side select transistor SDTr1’ and ‘first source side selecttransistor SSTr2’.

As shown in FIG. 12, the first source side select transistor SSTr1 andthe first drain side select transistor SDTr1 include a source sideconductive layer 41 a and a drain side conductive layer 41 b,respectively. The source side conductive layer 41 a functions as asource side select gate line SGS1 of the first source side selecttransistor SSTr1. The drain side conductive layer 41 b functions as adrain side select gate line SGD1 of the first drain side selecttransistor SDTr1.

As shown in FIG. 13, the source side conductive layer 41 a is formed soas to surround the semiconductor layer 48 a with a gate insulating film43 a interposed therebetween, and the drain side conductive layer 41 b,which is in the same layer as the source side conductive layer 41 a, isformed similarly so as to surround the semiconductor layer 48 b with agate insulating film 43 b interposed therebetween. The source sideconductive layer 41 a and the drain side conductive layer 41 b areconfigured by polysilicon (poly-Si).

Next, an erase operation in the nonvolatile semiconductor memory devicein accordance with the present embodiment is described with reference toFIGS. 14, 15A, and 15B. FIGS. 14 and 15A show an equivalent circuitdiagram of the memory cell array AR1 and voltages applied to each part.FIG. 15B is a timing chart showing timing of application of voltages. Itis assumed here too that, of the two sub-blocks in one memory block MB,the sub-block SB1 is set as erase target, and the sub-block SB2 is setto erase prohibit. Moreover, in the description below, the select gatelines SGD2 and SGS2 in the sub-block SB1 are referred to as SGD21 andSGS21, and the select gate lines SGD1 and SGS1 in the sub-block SB1 arereferred to as SGD11 and SGS11; similarly, the select gate lines SGD2and SGS2 in the sub-block SB2 are referred to as SGD22 and SGS22, andthe select gate lines SGD1 and SGS1 in the sub-block SB2 are referred toas SGD12 and SGS12.

In the sub-block SB1 selected as erase target, at time t1, the bit linesBL and source line SL are each set to the erase voltage Vera (about 20V). Meanwhile, the word lines WL are applied with the ground voltage Vss(0 V). Then, at time t3, the second drain side select gate line SGD21and the second source side select gate line SGS21 are each applied withthe voltage Vera-ΔV which is lower than the voltage Vera by about avoltage ΔV (for example, 5-8 V). This causes a GIDL (Gate Induced DrainLeakage) current to be generated at an end of the drain side selecttransistor SDTr2 on the bit line BL side and an end of the source sideselect transistor SSTr2 on the source line SL side in the sub-block SB1(refer to FIG. 15A), whereby the voltage Vera applied to the bit linesBL and source line SL is transferred to the body of the memory units MUin the sub-block SB1. This causes the erase operation in the sub-blockSB1 to be executed as a result of the potential difference between thevoltage Vera of the body and the voltage Vss of the word lines WL.Meanwhile, at time t3, the drain side select gate line SGD11 and thesource side select gate line SGS11 are applied with a voltage Vmidhaving a magnitude substantially intermediate between the erase voltageVera′ and the ground voltage Vss (for example, about 10 V).

In the unselected erase-prohibited sub-block SB2, at time t2, the drainside select gate line SGD22 and the source side select gate line SGS22are applied with the voltage Vera′ substantially identical to the erasevoltage Vera, whereby generation of a GIDL current is prevented. Inaddition, at time t2, the drain side select gate line SGD12 and thesource side select gate line SGS12 are applied with the voltage Vmidhaving a magnitude substantially intermediate between the erase voltageVera′ and the ground voltage Vss (for example, about 10 V). As a result,the difference in voltage applied between the plurality of linesdisposed adjacent to each other with a small wiring space is reduced.Hence, the risk of a GIDL current being generated can be reduced. Thatis, the risk of a false erase occurring in the unselected sub-block SB2can be reduced.

Note that, as shown in FIGS. 16A and 16B, the voltage applied to theselect gate lines SGD11 and SGS11 may be set to the voltage Vera-ΔVsimilarly to the select gate lines SGD21 and SGS21.

FIG. 17A is one example of a circuit for generating the above-mentionedvoltage Vmid. The circuit in FIG. 17A differs from that in FIG. 9A incomprising a level shifter 111, an NMOS transistor 112, splittingresistances 113 and 114, and a differential amplifier 115, forgenerating the voltage Vmid. The NMOS transistor 112 has its drainapplied with the voltage Vera′ and its source connected to one end ofthe splitting resistances 113 and 114. A voltage generated at the sourceis the voltage Vmid. The other end of the splitting resistances 113 and114 is grounded, and a connection node of the splitting resistances 113and 114 is connected to one input terminal of the differential amplifier115. An output terminal of the differential amplifier 115 is connectedto the level shifter 111, and an output terminal of the level shifter111 is connected to a gate of the NMOS transistor 112.

This circuit configuration causes the voltage Vmid generated at thesource of the NMOS transistor 112 to rise with substantially the sametiming as the voltage Vera′. In addition, determination of whether thisvoltage Vmid has reached a desired voltage is performed by the splittingresistances 113 and 114 and the differential amplifier 115. When thevoltage Vmid reaches the desired voltage, an output signal bEN2 of thedifferential amplifier 115 switches to “H”. This causes an output signalVout of the level shifter 111 to become “L”, whereby the NMOS transistor112 is switched off (OFF). Conversely, when the voltage Vmid falls belowthe desired voltage, the output signal Vout of the level shifter 111becomes “H”, whereby the NMOS transistor 112 is turned on (ON).Repetition of such an operation causes the voltage Vmid to be maintainedat a constant value.

One example of the row decoder 2A utilized in the present embodiment isshown in FIG. 17B. FIG. 17B differs from FIG. 9B in that, in FIG. 17B,transfer transistors 112 c are provided for supplying the voltage Vmidto the select gate lines SGD11 and SGD12. There is no need to controlselect/unselect of the memory string MS in the case of the select gatelines SGD11 and SGD12, hence there is no need to provide the select gatelines SGD11 and SGD12 with pull down transistors corresponding to thetransfer transistors 112 b that become necessary during unselect.

Third Embodiment

Next, a nonvolatile semiconductor memory device in accordance with athird embodiment is described.

FIG. 18A is a circuit diagram of an overall configuration of thenonvolatile semiconductor memory device in accordance with the thirdembodiment. In this third embodiment, similarly to in the secondembodiment, the memory unit MU, as well as comprising the second selecttransistors SDTr2 and SSTr2, also comprises the separate selecttransistors SDTr1 and SDTr1. However, in this third embodiment, contraryto in the second embodiment, the first drain side select gate line SGD1and the first source side select gate line SGS1 in each block are eachcommonly connected between a plurality of sub-blocks. Otherconfigurations and various operations are substantially similar to thosein the above-described embodiments. An erase operation similar to thosein the above-described separate embodiments can be performed by applyingvoltages of the kind shown in FIGS. 15A and 15B.

FIG. 18B shows one example of the row decoder 2A utilized in the presentembodiment. FIG. 18B differs from FIG. 17B in that, in FIG. 18B, onlyone transfer transistor 112 c is provided for supplying the voltage Vmidto the select gate lines SGD11 and SGD12.

Fourth Embodiment

Next, a nonvolatile semiconductor memory device in accordance with afourth embodiment is described.

FIG. 19A is a circuit diagram of an overall configuration of thenonvolatile semiconductor memory device in accordance with the fourthembodiment, and FIG. 19B is a modified example. In addition, FIG. 20 isa schematic perspective view of the memory cell array AR1. This fourthembodiment has one drain side select transistor SDTr and one source sideselect transistor SSTr for one memory unit MU, similarly to the firstembodiment. However, this fourth embodiment differs from the firstembodiment in having dummy memory transistors DMSS and DMDS providedbetween the drain side select transistor SDTr or source side selecttransistor SSTr and the memory transistors MTr. The dummy transistorsDMSS and DMDS configure part of the memory string MS and have astructure similar to that of ordinary memory transistors MTr, but arenot employed for data storage and have their threshold voltagemaintained at a constant value (for example, constantly at an eraselevel).

As shown in FIG. 21, the dummy transistor DMSS comprises: the memorygate insulating layer 33 formed so as to surround the columnar portion34 a of the memory semiconductor layer 34 similar to in the memorytransistors MTr; and a dummy word line conductive layer 31 e provided soas to surround the columnar portion 34 a sandwiching the memory gateinsulating layer 33 therebetween. The dummy word line conductive layer31 e is formed from polysilicon, for example, and functions as a dummyword line DWLS.

Similarly, the dummy transistor DMDS comprises: the memory gateinsulating layer 33 formed so as to surround the columnar portion 34 aof the memory semiconductor layer 34; and a dummy word line conductivelayer 31 e provided so as to surround the columnar portion 34 asandwiching the memory gate insulating layer 33 therebetween. The dummyword line conductive layer 31 e functions as a dummy word line DWLD.

An erase operation in this fourth embodiment can be executed in asubstantially similar manner to that in the second embodiment. That is,the erase operation can be executed in a sub-block unit by applying thevoltage applied to the first drain side select gate line SGD1 and firstsource side select gate line SGS1 in the second embodiment, as is, tothe dummy word lines DWLD and DWLS, and setting applied voltages to theother lines similarly to in the second embodiment.

That is, in the case of the configuration in FIG. 19A, it is onlyrequired to apply voltages of the kind shown in FIGS. 15A and 16A toeach part. In the case of a configuration in which the dummy word linesDWLD and DWLS are commonly connected between a plurality of sub-blocksSB as in the configuration in FIG. 19B, it is only required to applyvoltages of the kind shown in FIG. 16A to each part.

Fifth Embodiment

Next, a nonvolatile semiconductor memory device in accordance with afifth embodiment is described.

Configuration of the device is substantially similar to that in thesecond embodiment, and description thereof is thus omitted. However, asshown in FIG. 22A, this embodiment differs from the second embodiment inthat, in this embodiment, for example, voltages of each part, prior tobeing raised to the voltages Vera, Vera-ΔV, and so on, are first raisedto the voltage Vmid, and then raised to the target voltages Vera andVera-ΔV.

Note that, as shown in FIG. 22B, the voltages applied finally to theselect gate lines SGD11 and SGS11 may be set to the voltage Vmid inplace of the voltage Vera-ΔV.

FIG. 23 is one example of a charge pump circuit utilizable in thisembodiment. In this embodiment, timing of generation of the voltage Vmidis arbitrary, hence, as shown in FIG. 23, the voltage Vmid can begenerated using an independent oscillator 101′ and charge pump circuit102′.

Sixth Embodiment

Next, a nonvolatile semiconductor memory device in accordance with asixth embodiment is described with reference to FIGS. 24-26. FIG. 24 isa circuit diagram of an overall configuration of the nonvolatilesemiconductor memory device in accordance with the sixth embodiment.FIGS. 25 and 26 are, respectively, a schematic perspective view and across-sectional view of a memory cell array AR1 in the nonvolatilesemiconductor memory device in accordance with the sixth embodiment.Note that configurations similar to those in the first and secondembodiments are assigned with identical symbols to the first and secondembodiments and detailed descriptions thereof are omitted below.Moreover, although a structure of the memory cell array is shown inFIGS. 25 and 26 with a portion omitted, the structure of the memory cellarray is similar to that in the aforementioned embodiments.

This embodiment, as well as comprising the second drain side selecttransistor SDTr2 and the second source side select transistor SSTr2,also comprises a plurality (for example, two) of first drain side selecttransistors SDTr1 and SDTr1′, and a plurality (for example, two) offirst source side select transistors SSTr1 and SSTr1′, connected inseries to the second drain side select transistor SDTr2 and secondsource side select transistor SSTr2, respectively. The selecttransistors SDTr1 and SDTr1′ are connected in series between the selecttransistor SDTr2 and the memory string MS. The select transistors SSTr1and SSTr1′ are connected in series between the select transistor SSTr2and the memory string MS. Configurations of other portions aresubstantially identical to configurations in the second embodiment(FIGS. 11, 12, and 13), and repetitive descriptions are thus omitted.

Next, an erase operation in the nonvolatile semiconductor memory devicein accordance with the sixth embodiment is described with reference toFIGS. 27 and 28. Similarly to the description in the second embodiment,the case is described here where, of the two sub-blocks in one memoryblock MB, the sub-block SB1 is set as erase target, and the sub-blockSB2 is set to erase prohibit. The select gate lines SGD2 and SGS2 in thesub-block SB1 are referred to as SGD21 and SGS21, and the select gatelines SGD1, SGS1, SGD1′, and SGS1′ in the sub-block SB1 are referred toas SGD11, SGS11, SGD11′, and SGS11′, respectively. Similarly, the selectgate lines SGD2 and SGS2 in the sub-block SB2 are referred to as SGD22and SGS22, and the select gate lines SGD1, SGS1, SGD1′, and SGS1′ in thesub-block SB2 are referred to as SGD12, SGS12, SGD12′, and SGS12′,respectively.

Voltages applied finally to each part for the erase operation aresubstantially similar to those in the second embodiment. However, inthis embodiment, similarly to in the fifth embodiment, for example,voltages of each part, prior to being raised to the voltages Vera,Vera-ΔV, and Vera′, are first raised to an intermediate voltage Vmid1having a size substantially intermediate between the erase voltage Vera′and the ground voltage Vss, and then raised to the target voltages Vera,Vera-ΔV, and Vera′. Note that, similarly to in the second embodiment,raising to the intermediate voltage Vmid1 may be omitted and controlperformed such that the voltages are raised directly from the groundvoltage to the target voltages Vera, Vera-ΔV, and Vera′.

One memory string MS in this embodiment includes two first drain sideselect transistors SGD1 and SGD1′ connected in series, and two firstsource side select transistors SGS1 and SGS1′ connected in series.

In both the selected sub-block SB1 and the unselected sub-block SB2, thefirst drain side select transistors SGD1 (SGD11 and SGD12) and the firstsource side select transistors SGS1 (SGS11 and SGS12) are applied withthe voltage Vmid1, and the first drain side select transistors SGD1′(SGD11′ and SGD12′) and the first source side select transistors SGS1′(SGS11′ and SGS12′) are applied with a voltage Vmid2 (<Vmid1) smallerthan this voltage Vmid1 (refer to FIG. 28). As a result, the differencein voltage applied between the plurality of lines disposed adjacent toeach other with a small wiring space is further reduced compared to theprevious embodiments. Hence, the risk of a GIDL current being generatedcan be reduced.

FIG. 29 is one example of a charge pump circuit utilizable in thisembodiment. A circuit configured to generate the voltages Vera, Vera′,and Vera-ΔV shown in an upper portion of FIG. 29 has a configurationsimilar to that in FIG. 23. Moreover, a circuit shown in a lower portionof FIG. 29 is a circuit for generating the voltages Vmid1 and Vmid2.Configurative elements identical to those in the circuit in the lowerportion of FIG. 23 are assigned with identical symbols to FIG. 23 anddetailed descriptions thereof are omitted. Furthermore, in order togenerate the voltage Vmid2, the circuit in the lower portion of FIG. 29comprises a level shifter circuit 111′, an NMOS transistor 112′,splitting resistances 113′ and 114′, and a differential amplifiercircuit 115′. These are similar to the level shifter circuit 111, NMOStransistor 112, splitting resistances 113 and 114, and differentialamplifier circuit 115 shown in FIG. 17A, and detailed descriptionsthereof are thus omitted.

Note that the above description is a specific description ofconfiguration and operation in the case where there are two each of thefirst drain side select transistors SDTr1 and the first source sideselect transistors SSTr1 (SDTr1 and SDTr1′, and SSTr1 and SSTr1′).However, there is no need for the number of first drain side selecttransistors SDTr1 and first source side select transistors SSTr1 to betwo, and there may be three or more. In the case that there are n firstdrain side select transistors SDTr1(1), SDTr1(2), . . . , SDTr1(n)disposed in order from a side close to the bit line BL, the voltageVmid1 applied to a gate SGD1(1) of the select transistor SDTr1(1) is setto a largest value, and, thereafter, the further a select transistorSDTr is from the bit line BL, the smaller the value of the voltage Vmidapplied to its gate is set (Vmid1>Vmid2> . . . >Vmidin). Note that thevoltage applied to gates of the first drain side select transistors andfirst source side select transistors in the selected sub-block may beset to the voltage Vera-ΔV in place of Vmid1, Vmid2, . . . , Vmidn,similarly to in the modified example in the second embodiment (FIG.16A).

Seventh Embodiment

Next, a nonvolatile semiconductor memory device in accordance with aseventh embodiment is described with reference to FIGS. 30-33. FIG. 30is a circuit diagram of an overall configuration of the nonvolatilesemiconductor memory device in accordance with the seventh embodiment.FIG. 31 is a schematic perspective view of a memory cell array AR1 inthe nonvolatile semiconductor memory device in accordance with theseventh embodiment. Note that configurations similar to those in thefirst and second embodiments are assigned with identical symbols to thefirst and second embodiments and detailed descriptions thereof areomitted below.

This embodiment has the feature of including dummy transistors betweenthe select transistors SDTr2 and SSTr2 and the memory transistors MTr1and MTr8, similarly to the fourth embodiment. However, this embodimentdiffers from the fourth embodiment in having a plurality (for example,two) of the dummy transistors connected in series to one selecttransistor SDTr2 (or SSTr2). Specifically, two dummy transistors DMDS2and DMDS1 are connected in series to the drain side select transistorSDTr2. In addition, two dummy transistors DMSS2 and DMSS1 are connectedin series to the source side select transistor SSTr2. The dummytransistor DMDS1 is connected in series to the memory transistor MTr8.The dummy transistor DMDS2 is connected in series to the dummytransistor DMDS1 and has one end connected to the drain side selecttransistor SDTr2. The dummy transistor DMSS1 is connected in series tothe memory transistor MTr1. The dummy transistor DMSS2 is connected inseries to the dummy transistor DMSS1 and has one end connected to thesource side select transistor SSTr2. Configurations of other portionsare substantially identical to configurations in the fourth embodiment,and repetitive descriptions are thus omitted.

Next, an erase operation in the nonvolatile semiconductor memory devicein accordance with the seventh embodiment is described with reference toFIGS. 32 and 33. Similarly to the description in the sixth embodiment,the case is described here where, of the two sub-blocks in one memoryblock MB, the sub-block SB1 is set as erase target, and the sub-blockSB2 is set to erase prohibit. Note that, in FIG. 32, the dummytransistors DMDS2, DMDS1, DMSS2, and DMSS1 in the sub-block SB1 arereferred to as DMDS21, DMDS11, DMSS21, and DMSS11, respectively.Moreover, the dummy transistors DMDS2, DMDS1, DMSS2, and DMSS1 in thesub-block SB2 are referred to as DMDS22, DMDS12, DMSS22, and DMSS12,respectively.

Voltages applied finally to the bit lines BL, source line SL, and selectgate lines SGD2 and SGS2 for the erase operation are substantiallysimilar to those in the sixth embodiment. Moreover, voltages applied todummy word lines DWLD21, DWLS21, DWLD11, DWLS11, DWLD22, DWLS22, DWLD12,and DWLS12 of the dummy transistors are identical to voltages applied tothe select gate lines SGD11, SGS11, SGD11′, SGS11′, SGD12, SGS12,SGD12′, and SGS12′ in the sixth embodiment. This allows similaradvantages to be displayed to those in the sixth embodiment. Note thatthe voltage applied to the dummy word lines DWLD21 and DWLS21 in theselected sub-block SB1 may be set to the voltage Vera-ΔV in place of thevoltage Vmid1, similarly to in the modified example in the secondembodiment (FIG. 16A).

Furthermore, the fact that the number of dummy transistors DMDS and DMSSneed not be two, and may be three or more is similar to the selecttransistors SDTr1 and SSTr1 in the sixth embodiment. The fact that, atthis time, the closer a dummy transistor DMDS or DMSS is to the bit lineBL, the larger the value of the voltage Vmid applied to its gate is set,and the further a dummy transistor DMDS or DMSS is to the bit line BL,the smaller the value of the voltage Vmid applied to its gate is set issimilar to the sixth embodiment. In addition, the charge pump circuitshown in FIG. 29 may be utilized for these erase operations.

While certain embodiments of the inventions have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the inventions. Indeed, the novel methodsand systems described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in theform of the methods and systems described herein may be made withoutdeparting from the spirit of the inventions. The accompanying claims andtheir equivalents are intended to cover such forms or modifications aswould fall within the scope and spirit of the inventions.

For example, each of the above-described embodiments describes anexample including a memory cell array AR1 which has U shaped memorystrings MS arranged therein. However, the above-described embodimentsare not limited to this and may employ, for example, I shaped memorystrings having all the memory transistors arranged in one straight line.

Moreover, in the above-described embodiments, the select transistorsSDTr and SSTr differ from the memory transistors MTr in being configuredas transistors which have a gate insulating film formed from a one-layerfilm of silicon oxide, that is, which lack the charge storage layer 33b. However, the present invention is not limited to this configuration,and the select transistors SDTr and SSTr may be configured to includethe memory gate insulating layer 33 comprising the three-layer structureof the block insulating layer 33 a, charge storage layer 33 b, andtunnel insulating layer 33 c, similarly to the memory transistors.

What is claimed is:
 1. A memory device, comprising: a memory cell arrayincluding a first memory block, the first memory block including a firstsub block and a second sub block, the first sub block including a firstmemory unit and a second memory unit, the first memory unit including afirst drain side selective transistor, a plurality of first memorycells, a first source side selective transistor, a plurality of secondmemory cells, and a first connecting portion that connects the firstmemory cells and the second memory cells in series, the second memoryunit including a second drain side selective transistor, a plurality ofthird memory cells, a second source side selective transistor, aplurality of forth memory cells and a second connecting portion thatconnects the third memory cells and the fourth memory cells in series;the second sub block including a third memory unit and a fourth memoryunit, the third memory unit including a third drain side selectivetransistor, a plurality of fifth memory cells, a third source sideselective transistor, a plurality of sixth memory cells and a thirdconnecting portion that connects the fifth memory cells and the sixthmemory cells in series; the fourth memory unit including a fourth drainside selective transistor, a plurality of seventh memory cells, a fourthsource side selective transistor, a plurality of eighth memory cells anda fourth connecting portion that connects the seventh memory cells andthe eighth memory cells in series; a first bit line connected to thefirst drain side selective transistor and the third drain side selectivetransistor; a second bit line connected to the second drain sideselective transistor and the fourth drain side selective transistor; aplurality first word lines connected to gates of the first memory cells,gates of the third memory cells, gates of the fifth memory cells, andgates of the seventh memory cells, respectively, a plurality second wordlines connected to gates of the second memory cells, gates of the fourthmemory cells, gates of the sixth memory cells, and gates of the eighthmemory cells respectively; a first source side select gate lineconnected to a gate of the first source side selective transistor and agate of the second source side selective transistor; a second sourceside select gate line connected to a gate of the third source sideselective transistor and a gate of the fourth source side selectivetransistor; and a control circuit which applies a first voltage to thefirst bit line and the second bit line, a second voltage which is lowerthan the first voltage to the first source side select gate line, athird voltage which is higher than the second voltage to the secondsource side select gate line, a fourth voltage which is lower than thesecond voltage to the plurality of first word lines and the plurality ofsecond word lines, while erase operation to the first sub block isoperated.
 2. The memory device according to claim 1, wherein the firstsource side select gate line and the second source side select gate lineare electrically disconnected from each other and capable to apply avoltage independently.
 3. The memory device according to claim 1,wherein one of the first word lines and a corresponding one of thesecond word lines are disposed in the same layer, respectively, and theone of the first word lines and the corresponding one of the second wordlines are electrically disconnected from each other.
 4. The memorydevice according to claim 1, wherein the first connecting portion, thesecond connecting portion, the third connecting portion, and the fourthconnecting portion include poly silicon.